OpenTTD is an open source simulation game based upon the popular Microprose game "Transport Tycoon Deluxe", written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.
OpenTTD takes an already awesome simulation game and brings it up to date. Adds a level of realism that was always missing but without making the game unecessarily dry.
Custom graphics, custom AI, and pretty custom anything you need. What a game...